MASER FOR FREQUENCIES IN THE 7{14 20 GHz RANGE

ABSTRACT

A traveling wave maser (TWM) of the comb type is disclosed for operation in the 7-20 GHz frequency range. The TWM is loaded with 0-degree Czochralski grown ruby with its C-axis at angle theta 90* with respect to the magnetic field. The TWM is pumped in the push-push mode between levels 1-3 and 3-4 with two different frequencies.

United States Patent Low et a1.

[is] 3,676,787 [451 July 11,1972

[54] MASER FOR FREQUENCIES IN THE 7-20 GHZ RANGE [72] Inventors: GeorgeM. Low, Acting Administrator of the National Aeronautics and SpaceAdministration with respect to an invention of; Robert C. Clauss; Rex B.Quinn, both of La Crescenta, Calif.

[22] Filed: Aug. 11, 1970 [2]] Appl. No.: 63,144

[52] U.S. Cl ..330/4, 331/94 [51] Int. Cl ..l-l0ls 1/00 [58] FieldofSearch ..33l/94 [56] References Cited UNITED STATES PATENTS 3,214,70110/1965 Fang-Shang Chen et a1 ..330/4 3,486,123 12/1969 Paine ..330/43,343,102 9/1967 Scovil ..330/4X FOREIGN PATENTS OR APPLICATIONS 254,4469/1964 Australia ..330/4 OTHER PUBLICATIONS Microwave Solid State Masersby A. E. Siegman, McGraw- Hill, 1964, pp. 292- 294 Maser Action in Rubyby Off Resonance Pumping, F. Arams et al., Proceedings of the IRE, Sept.1961, pp. 1426- 1427 Primary ExaminerBenjamin A. Borchelt AssistantExaminer-H. A. Birmiel Atlorney-J. H. Warden, Paul F. McCaul and John R.Manning [57] ABSTRACT A traveling wave maser (TWM) of the comb type isdisclosed for operation in the 7-20 GHz frequency range. The TWM isloaded with O-degree Czochralski grown ruby with its C-axis at angle0=90 with respect to the magnetic field. The TWM is pumped in thepush-push mode between levels l-3 and 3-4 with two differentfrequencies.

9 Claims, 5 Drawing Figures i SIGNAL Patented July 11, 1972 FREQUENCY.GHZ

Sheets-Sheet 1 FIG. I

SI GNAL CONVENTIONAL GENERAToR TWM REcEIvING I l I EQUIPMENT l2 l3 IO l4I's PUMP uNIT 42.9 I l I l A I I I 2 23.9 b H= 36006 H: 4900 g PUSH-PUSH2L6 53 PuMPTRANsITIoN 47 2|.5 PUSH-PULL PUMP TRANsITIoNs 52 PUSH-PUSH IPUMP TRANSITION I9.0 A

46 85 h SIGNAL FREQUENCY GH AT 84 8273 5| e= 54.73 SIGNAL FREQUENCY9.29? GHz AT e=9o 6 deg IIN'VEAWTORS ROBERT C. CLAUSS Fl (5. 4- RE B.QUINN ATTORNEYS July 11, 1972 3,676,787

3 Sheets-Sheet 2 FIG.2

GHz

INVENTORS ROBERT C. CLAUSS REX B. QUINN MASER FOR FREQUENCIES IN THE7-20 GHZ RANGE The invention described herein was made in theperformance of work under a NASA contract and is subject to theprovisions of Section 305 of the National Aeronautics and Space Act of1958, Public Law 85-568 (72 Stat. 435; 42 USC 2457).

BACKGROUND OF THE INVENTION 1. Field of the Invention The presentinvention is generally directed to a maser amplifier and, moreparticularly, to a maser amplifier of the comb type with improvedgain-bandwidth product and noise-temperature characteristics.

2. Description of the Prior Art The construction and use of a travelingwave maser (TWM) of the comb type is well known. Such TWMs are describedin the literature and in many US. Pats, including U.S. Pat. Nos.3,214,701, 3,296,541, 3,299,364 and 3,486,123, the latter having beenissued to one of the applicants of the present application. As is known,in each of these TWMs, an active material is employed. In US. Pat. No.3,486,123 the active material is in the form of slabs cut fromzero-degree Czochralski rube with their lengths along the C-axis of theruby.

The use of ruby in TWMs is well known. For example, the advantages ofruby and its energy level diagrams are described in Microwave SolidState Masers by A. E. Siegman, Library of Congress Catalog Card Number63-l2l3l, particular attention being directed to pages 1 l5 and 116. Asstated therein, an angle generally defined as 0 of 5444 54.7 between theruby C-axis and the magnetic field is extremely convenient in certaindoubly-pumped masers schemes. This angle value, i.e., 0 54.7 is known asthe double-pump angle. A similar reference is found in Technical ReportNo. 457 of the Lincoln Laboratory of Massachusetts Institute ofTechnology entitled, The Microwave System of the Haystack PlanetaryRadar by CW. Jones, published Oct. 23, 1968.

Although TWMs with ruby cut at 0 54.7 have been constructed, theirconstruction presents several problems, primary among them being that ofaccurate ruby orientation. It has been determined that any minordeviations from the desired value of 0, either due to improper rubygrowth, cutting or due to imperfect alignment during construction orduring subsequent use results in significant deviations in the pumpingfrequency required for a particular signal frequency. The signalfrequency, in a fixed magnetic field, also experiences significantdeviation due to crystal misalignment. Such deviations severely degradethe amplification factor of the maser.

Herebefore, two basic pumping techniques have been used in conjunctionwith a TWM with ruby (one at 0 54.7 and another at 0 90) and push-pullpumping, which is referred to in the afore-mentioned references asdouble pumping (for 0 54.7).

In developing a TWM for a range of frequencies from about 7 GHz to 20GHz, as well as in designing a TWM for lower frequencies in a lowerband, referred to as the S-band, it has been discovered that the effectsof ruby misalignment are greatly reduced when 6 90, i.e. when the C-axisis perpendicular to the magnetic field. Indeed, deviations from 6 90 upto l produced only small changes in the pump and signal frequencies.However, in the range of 7 GHz to 20 GHz with ruby at 0 90, prior artpumping techniques have been found to be quite unsatisfactory.Performance, as represented by measured inversion ratio, was found todegrade as the signal frequency increased above 8.5 GHz. A needtherefore exists for a new method of pumping a TWM with ruby at 6 90 toproduce high amplification factors at frequencies above 7 GHz.

OBJECTS AND SUMMARY OF THE INVENTION It is a primary object of thepresent invention to provide a new improved TWM for frequencies in the 720 GHz range.

Another object of the present invention is to provide a TWM with ruby asthe active material, which is simpler to construct than prior. art TWMs,and which extends the signal frequency range to about 20 GHz.

A further object of the present invention is to provide a new method ofpumping a TWM with ruby oriented with respect to the magnetic field soas to minimize changes in the pump and signal frequencies due to rubymisalignment.

These and other objects of the present invention are achieved byproviding a TWM with ruby at 0 which is pumped in the push-push mode,with two separate pump frequencies rather than in the single frequencymode (used at 6 90), or push-pull mode, (used with ruby at 0 54.7"). Ithas been discovered that unusual and unexpected results, represented byhigh measured inversion ratios, are achievable when push-push pumpingwith two pump frequencies is employed with a TWM operable in the 7 to 20GHz range with the ruby oriented at 0 90. It is submitted that push-pushpumping per se is known and is not regarded as new. However, heretofore,push-push pumping was only performed with a single frequency source. Thenovelty of the present invention resides in the use of a push-pushpumping technique in connection with a TWM, operating above 7 GHz and inparticular, such a TWM in which ruby at 0 90 is employed as the activematerial and in which two, rather than a single, pumping frequencies areemployed. That is, whereas in the prior art only single frequency (at 090) or push-pull pumping (at 0 54.7), were used with ruby, in thepresent invention pushpush pumping is employed for a TWM with ruby at 090, by the use of two separate pump frequency sources for operation. Ithas been discovered that such a TWM, between 7 and 20 GHz, in additionto simplified construction due to the reduced alignment problems andimproved dynamic stability, has improved tunability, lower noisetemperature and higher gain than prior art designs.

The novel features of the invention are set forth with particularity inthe. appended claims. The invention will be best understood from thefollowing description when read in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a simplified block diagramof the present invention;

FIG. 2 is a cross sectional view of the TWM of the present invention;and

FIGS. 3 5 are diagrams used to highlight the mode of operation and theadvantages of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention may bestbe introduced in conjunction with FIGS. 1-3. In FIG. 1, numeral 10designates a TWM which is shown connected to a signal generator 12 viaan input line 13, through which the signal to be amplified is suppliedto the TWM l0, and an output line 14, through which the amplified signalis supplied to conventional receiving equipment 15. Also shown is a pumpunit 16 which supplies the pump energy to TWM 10. As briefly stated andas will be explained hereafter, unit 16 pumps TWM 10 in a push-push modewith two separate pump frequencies which accounts for the unexpectedadvantageous results of the present invention.

TWM 10 is assumed to be of the comb type. Structurally, it is similar tothe TWM shown and described in US Pat. No. 3,486,l23 with only minordesign changes introduced to optimize its performance in the X-band.FIG. 2 represents a cross-sectional view of TWM 10 and is analogous tothe crosssectional view shown in FIG. 4 of US. Pat. No. 3,486,123. InFIG. 2, numeral 20 represents the main body of the TWM, with a dividingwall 22, numerals 24 designate the comb fingers, and numerals 26 theactive ruby material. Numerals 28 designate the isolators, which arestrips of polycrystalline yittrium-iron-garnet (YIG), mounted on aluminasupports 30. As seen, the ruby material 26 is located on only one sideof the comb fingers 24, while an alumina support material 30 is locatedon the outside of the comb fingers. Alumina pads 32 space the isolators28 away from the base of the comb body 20 to prevent the isolator strips28 from contacting the comb body 20.

As previously stated typically in the prior art, the ruby 26 is cut sothat its C-axis is at 54.7 with respect to the direction of the magneticfield, which is designated in FIG. 2 by arrow 35. This is particularlythe case when double-pumping (generally referred to as push-pullpumping) is employed. This axis orientation however is quite critical.It must be accurately achieved and maintained to avoid pump and signalfrequency changes. It has been discovered that crystal orientationproblems can be greatly simplified and dynamic stability greatlyincreased by choosing 6 90. That is, by placing the rubys C-axis in aplane which is perpendicular to that of the magnetic field direction.This plane is represented in FIG. 2 by arrow 36 and a directionperpendicular to the plane of the Figure as represented by dot 37.

The ruby orientation with 6 90 simplifies the construction of the TWM,and increases its dynamic stability. However, it presents pumpingproblems when operated above 8.56 GI-lz with prior art pumpingtechniques. These pumping problems are solved by the present invention.When operating above 8.5 GHZ with ruby with 0 90 attempts were made topump the TWM with known pumping methods. Basically, these include singlefrequency and push-pull pumping. However, the performance was found tobe unsatisfactory due to very low measured inversion ratios. Experimentsindicated that performance degrades as the signal frequency increasesfrom 7 to 20 Gl-Iz. However, it was discovered that high performance isachieved when push-push pumping with two separate pump frequencies isemployed.

FIG. 3, to which reference is now made is a diagram of the energy levelsof ruby. The four levels are designated by numerals 1 through 4. It hasbeen discovered that by pumping the TWM 10 with ruby at 6 90 with twoseparate frequencies between levels l-3 and 34, as represented by arrows41 and 42 high gain is attained between levels 1 and 2, as representedby arrow 43.

The advantages of orienting the ruby with 0 90 are best highlighted inconjunction with FIG. 4. Therein lines 45-47 designate changes of signalfrequency and two push-push pump transitions for a particular embodimentactually reduced to practice as a function of ruby orientation with 090. As seen, misorientations of 1 can be tolerated with little effect inthe signal frequency or the pump transitions. On the other hand,significant changes in the signal frequency and the pump transitionsoccur due to misorientations of the ruby when 6 54.7 and conventionalpush-pull pumping is employed. These changes are represented by lines51-53.

The advantages of the present invention may be further highlighted bythe results of various experiments summarized in chart form in FIG. 5.As seen therefrom, when 0 90, except near the lower end of the 7 20GI-Iz range, only when push-push pumping is employed are high inversionratios attained. For example, with a signal frequency of 8.5 GHz andsingle frequency pumping between levels 1 -4 is employed, an inversionratio of 2.5 was achieved. However, under the same pumping conditionwith a signal frequency of 16.8 GHz, the measured inversion ratio isextremely low, being 0.l. Similarly with 0 90, single frequency of 16.8and with pushpull pumping between transitions 1-3 and 2-4 withfrequencies of 37.4 GHz and 26.5 Gl-lz the measured inversion ratio isonly 0.35. On the other hand, the highest measured inversion ratios areattained as long as push-push pumping (between levels I-3 and 3-4) isemployed.

It is thus seen that with ruby at 6 90 with a signal frequency of 16.8GI-Iz satisfactory performance is only obtained with push-push pumpingrather than with either of the prior art pumping methods.

It should be noted that in the present invention either pump frequencyis higher than the signal frequency.

The TWM system of the present invention in which ruby is oriented with0= and is pumped by the push-push method, in addition to simplifiedcrystal orientation and increased dynamic stability has improvedtunability, lower noise tem perature and higher gain than previousdesigns. These advantageous properties are due to favorable transitionprobabilities which exist in ruby near 0 90. Combined with a successfulpumping technique, the resultant capability to provide a highamplification factor also improves noise temperature and tunability of amaser amplifier.

In one actual reduction to practice a 3 inch comb type TWM was operatedat 8.5 GHZ with ruby at 0 90 with pushpush pumping with two pumpingfrequencies (see third line of FIG. 5). The measured inversion ratio was3.2, with an electronic gain of more than 30dB. The tunable range wasfrom 7.6 GHZ to 8.5 GI-lz.

A dual TWM (two 3 inch combs, side by side) was also constructed and isin actual field operation. It is tunable from 7.6 GHZ to 8.9 Gl-Iz. Theequivalent input noise temperature was measured to be 7 K at 8450 MHz.Net gain between 30dB and 40dB is obtained by single frequency pumpingof the l-4 transition. However, a lOdB improvement is obtained when thesame TWM is pumped in the push-push mode with two separate pumpingfrequencies.

It should be noted that the use of ruby at 0 90 in the 7 to 20 Gl-Izrange with push-push pumping need not be restricted to masers ofcomb-type design. The above technique will produce improvements inperformance of any slow-wave structures, or waveguides filled with masermaterial, or masers of the cavity type.

There has accordingly been shown and described herein a new improved TWMfor operation between 7 and 20 GHz. It is appreciated that althoughparticular embodiments of the invention have been described andillustrated herein, it is recognized that modifications and variationsmay readily occur to those skilled in the art and, consequently, it isintended that the claims be interpreted to cover such modifications andequivalents.

What is claimed is: 1. In combination with a traveling wave maserdesigned for operation above 7 Gl-Iz and including an active masermaterial oriented therein with its C-axis in a plane perpendicular tothe magnetic field of the traveling wave maser, the combinationcomprising:

a signal source coupled to said traveling wave maser for supplying itwith signals to be amplified in said range; and

pump means coupled to said traveling wave maser for pumping it in thepush-push mode, with two distinct and separate pumping frequencies.

2. The arrangement as recited in claim 1 wherein said active masermaterial is zero-degree Czochralski grown ruby.

3. The arrangement as recited in claim 1 wherein said active masermaterial has four energy levels definable as levels 14 and said pumpmeans pumps said traveling wave maser in the push-push mode betweenlevels 1 and 3 and levels 3 and 4 with the signal being amplifiedbetween levels 1 and 2.

4. The arrangement as recited in claim 3 wherein the signal frequency isless than either pump frequency.

5. The arrangement as recited in claim 3 wherein said active masermaterial is zero-degree Czochralski grown ruby.

6. The arrangement as recited in claim 5 wherein said traveling wavemaser is pumped by two different frequencies and the signal frequency isless than either pump frequency.

7. In combination with a traveling wave maser of the comb structuredesigned for operation in a range of frequencies including the range of10 GHz to 20 GHz and including an active maser material oriented thereinwith its C-axis in a plane perpendicular to the magnetic field of thetraveling wave maser, the combination comprising:

a signal source coupled to said traveling wave maser for supplying itwith signals in said range to be amplified therein; and

maser material is zero-degree Czochralski grown ruby having four energylevels definable as levels 1-4 and said pump means pumps said travelingwave maser in the push-push mode between levels 1 and 3 and levels 3 and4 with the signal being amplified between levels 1 and 2.

1. In combination with a traveling wave maser designed for operationabove 7 GHz and including an active maser material oriented therein withits C-axis in a plane perpendicular to the magnetic field of thetraveling wave maser, the combination comprising: a signal sourcecoupled to said traveling wave maser for supplying it with signals to beamplified in said range; and pump means coupled to said traveling wavemaser for pumping it in the push-push mode, with two distinct andseparate pumping frequencies.
 2. The arrangement as recited in claim 1wherein said active maser material is zero-degree Czochralski grownruby.
 3. The arrangement as recited in claim 1 wherein said active masermaterial has four energy levels definable as levels 1-4 and said pumpmeans pumps said traveling wave maser in the push-push mode betweenlevels 1 and 3 and levels 3 and 4 with the signal being amplifiedbetween levels 1 and
 2. 4. The arrangement as recited in claim 3 whereinthe signal frequency is less than either pump frequency.
 5. Thearrangement as recited in claim 3 wherein said active maser material iszero-degree Czochralski grown ruby.
 6. The arrangement as recited inclaim 5 wherein said traveling wave maser is pumped by two differentfrequencies and the signal frequency is less than either pump frequency.7. In combination with a traveling wave maser of the comb structuredesigned for operation in a range of frequencies including the range of10 GHz to 20 GHz and including an active maser material oriented thereinwith its C-axis in a plane perpendicular to the magnetic field of thetraveling wave maser, the combination comprising: a signal sourcecoupled to said traveling wave maser for supplying it with signals insaid range to be amplified therein; and pump means coupled to saidtraveling wave maser for pumping it in the push-push mode, with twodistinct and separate frequencies.
 8. The arrangement as recited inclaim 7 wherein the signal frequency is less than either pump frequency.9. The arrangement as recited in claim 7 wherein said active masermaterial is zero-degree Czochralski grown ruby having four energy levelsdefinable as levels 1-4 and said pump means pumps said traveling wavemaser in the push-push mode between levels 1 and 3 and levels 3 and 4with the signal being amplified between levels 1 and 2.